1. heterosexual
2. used to describe a person who does not participate in "dangerous" activity such as drugs, alchohol, sex or criminal activity. A good girl/guy.
3. to tell it straight - to tell the truth
4. see straight up

1. Did you see how he reacted to that guy coming on to him? He is so straight.
2. Sally is so straight, she never breaks any rules.
3. "You being straight?"
"Seriously, it really happened."

There are multiple meanings for straight-
1. Traditional meaning: Without a bend or curve, evenly formed or set, not crooked.
2. Traditional meaning: Figurative; a) frank, honest, upright, b) right, correct
3. Traditional meaning: in proper order (numeric, chronologic, alphabetic, etc...),
4. Traditional meaning: without interruption or break; continuous, in an unbroken series, without reservations or exceptions,
5. Traditional meaning: unmixed, undiluted (reference to alcohol, or other substances, without qualification, unmodified,
6. Traditional meaning: showing no emotion or humor, serious rather than comic, natural rather than eccentric,
7. Traditional meaning: reliable, sure, authoritative,
8. Traditional meaning: behaving in a conventional manner, holding orthodox views, conventional, standard, normal,
9. Traditional meaning: made up of a set of five cards, as in poker,
10. Slang: term used in agreement, approval,
11. Slang: term used to qualify one's sexual preference,
12. Slang: term used to denote the validity of a statement or claim,
13. Slang: term used to describe a manner of frank communication, i.e.- straight talk,
14. Slang: term used to determine level of understanding, agreement, or otherwise mutual understanding and agreement

Traditionally the word Straight referenced the quality of an object posessing no bends, curves, or twists.
Today the word Straight refers to myriad categories including heterosexuality, commonality of opinion, earnest communication, and level of comprehension.